###### Cover of the book

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*Sudoku Pencil Pals* is a book of 250 moderately challenging to difficult
puzzles. We have pre-calculated (penciled) the candidate numbers for each cell,
row, column, and box to save you time when employing advanced techniques (like
the unique corner).

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We have self-published the book for the Amazon Kindle since Amazon makes it so easy. One
won't necessarily need a Kindle to view the book; Amazon offers free readers, like
Kindle for PC. But we do like the real Kindle.

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Here is a sample page from the book containing puzzle #0, our example puzzle:

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The book's glossary explains many solving techniques in a concise, semi-formal manner.
Here is the entry for the **unique rectangle**, which explains the unique corner
logic:

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**Unique rectangle** --- If the digit pair k1,k2 are the only possibilities in all four cells
at the intersections of two rows and two columns in two boxes, then the puzzle has two solutions
(and is not a proper Sudoku) because k1 and k2 may be freely interchanged to obtain the two solutions.
This is the *deadly pattern*.

When one additional digit k is present in one of the cells, then a **unique corner** has been found and k
may be entered there so as to avoid the deadly condition.

When one additional digit k is present in two adjacent corners (a **unique side**) of the unique
rectangle, then those corners form a conjugate pair of cells for digit k, and k may be eliminated
from possibilities in the row or column containing the two corner cells. Additionally, if those
corners are in the same box then k may be eliminated from other cells in the box. Other exploitations
of the unique rectangle are known, but uncommon.

This deadly pattern is but one of the many possible. Consider a completed Sudoku grid - all 81 entries
are present. The clues must be chosen so as to leave no deadly pattern exposed or the solution will
not be unique. If we were to exchange of all the 1 digits with all the 9 digits, we have a deadly
pattern of 18 cells, and one of the cells must have a clue to prohibit the free exchange of the two
digits. (The deadly patterns are *unavoidable sets* for Sudoku. There are hundreds of these
overlapping sets for each completed grid, and each set must have at least one clue to preserve
uniqueness.)

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Here is the link at Amazon.com if you would care to purchase the book ($9.99):

Sudoku Pencil Pals